Information signals are commonly exchanged between circuit cards in data processing equipment and a backplane board into which transmitting and receiving elements are plugged. Conventional backplanes utilize electrical interconnections to complete the communication paths that run between the receivers and transmitters of different processors. As long as data rates are relatively low, or the paths are massively parallel, electrical connections can provide satisfactory communication. However, as data rates increase, problems begin to appear due to cross-talk, skew, rf interference, losses, ground-loop isolation, impedance mismatch and latency. The exact data rate where these problems become critical varies with the application, but it often will occur at data rates between 10 MHz and 100 MHz. Between the upper and lower data rates of this range, it is possible to employ either an electrical backplane or an optical backplane for board-to-board communication links. Proposed Pi and Futurebus+ data busses are intended to operate within this range.
In order for an optical backplane to be advantageously used in place of a traditional electrical backplane, it must be competitive in cost, it must not be overly complex and it must offer future growth potential. Optical backplane designs are generally one of two types. They are of either a guided or a free-space design. The guided method may be accomplished either through optical fibers or through integrated optic waveguides. The free-space approach can be achieved either through microoptic elements or through a holographic lens systems. Some of the general advantages and disadvantages of each method are described below.
Guided Methods. The use of optical fibers for card-to-card communications typically requires a number of expensive transmitters and receivers, as well as fiber-to-fiber connections. Optical fibers must maintain alignment under all environmental conditions, including especially temperature and vibration. Arrangement of the optical fibers into a bus architecture will minimize the number of transmitters and receivers needed, but a fiber optic alignment will still be required. In addition, fiber optic bus architecture usually utilizes either a star-type configuration or a bus structure with a number of optical taps. Splitting losses on such a bus can unfavorably limit the number of cards that can be used with the bus.
Embedding optical fibers into an electrical backplane poses a compatibility problem and often requires that the fiber optics be formed into tight turns which can damage the fibers and are difficult to implement. The use of optical fibers also often involves labor-intensive preparation and fusion or mechanical splicing of the ends of the fibers.
IBM Technical Disclosure Bulletin, Vol. 21, No. 4, Sep., 1978, illustrates one example of an optical circuit module connector in which fiber optics communicate with a mix of optical and electrical components that are packaged upon a conventional card-on-board environment. Light-emitting-diodes (LEDs) or photodiodes are placed on a substrate that is mounted at right angles to the circuit modules, and the optical fiber connector carries the optical fibers into alignment with the LED or photodiodes.
Integrated waveguides are another way to provide guided wave communication which have the advantages of being fabricatable by batch methods, and they are rugged and can withstand relatively high temperatures. Some integrated waveguide implementations, however, are not designed for use with bus architecture, but rather they rely on parallel data flow. For bus operation either integrated taps or star couplers that are superior to the presently available taps or couplers must still be developed to provide practical integrated waveguide bus architectures.
Integrated waveguide development has generally been directed to massively parallel computing systems in which there are a multitude of parallel interconnection paths between processors on the same card rather than to card-to-card communication. This effort has resulted in the development of low-loss polymer waveguides with cross-overs, right-angle bends and connections to transmitters or receivers. Such devices at the present time still have excessive signal losses.
Free Space. There are several approaches that utilize free-space communication between cards. One such method uses optical-to-electrical relays at each board. The signal is converted to an optical signal and is sent to an adjacent card where it is converted to an electrical signal for use at that card. If it is to be transmitted further, another electrical-to-optical conversion is made and the data is sent to the next card. The disadvantages of this system include (1) the data may be corrupted by the number of electrical-to-optical and optical-to-electrical conversions that must be made, and (2) a clear path must be maintained in the card stack for data transmission for each clear channel.
Holographic lenses have been used for diffracting a beam to a number of receivers, but this type of lens is critically dependent upon the laser frequency. A very precisely controlled signal frequency over the entire temperature range of operation is required in order for the optical transmitter to work with the holographic lens.
Graded-index (GRIN) lenses are often used to couple one single mode optical fiber to another single mode optical fiber wherein a beam is launched by one GRIN lens that passes either through free space or through a combination of free space and an intervening optical element into another GRIN lens. In some cases the two GRIN lenses have been placed so the face of one GRIN lens abuts the face of the other GRIN lens. Examples that show the use of GRIN lens are found in U.S. Pat. No. 4,701,011, issued Oct. 20, 1987 to William Emkey, et al, entitled "Multimode Fiber-Lens Optical Coupler;" U.S. Pat. No. 4,817,205, issued Mar. 28, 1989 to Charles K. Asawa, entitled "Bimodal Optical Fiber Communication System Using Graded Index Fiber;"U.S. Pat. No. 5,050,954, issued Sep. 24, 1991, entitled "Multiport Optical Devices" to William B. Gardner, et al; and in U.S. Pat. No. 4,239,330, entitled "Multiple Optical Switch," issued Dec. 16, 1980 to Arthur Ashkin, et al.
U.S. Pat. No. 4,208,094, issued Jun. 17, 1980, entitled "Optical Switch" in the name of Walter J. Tomlinson, III, et al., disclosed a GRIN lens and a rotator reflecting surface that was mounted adjacent to one surface of the lens at an angle. A plurality of optical fibers are positioned at the other end of the lens. Rotation of the reflecting surface about the axis of the lens changes the coupling of light between the input end and the output fibers. The output fibers are retained in alignment in a V-groove that was formed in a retaining block for the fibers.
GRIN lenses are optical glass rods which commonly have a radial refractive index gradient that approximates a parabolic function. Therefore, waves traveling through the center of the lens are the slowest, whereas waves traveling at a distance displaced from the center of the lens are propagated faster in proportion to the distance that they are from the center of the lens. In GRIN lenses the light beams are alternately collimated and focused at repetitive intervals as the beam advances through the lens.
A pitch of 1 for a GRIN rod lens is defined as the distance between three successive collimating planes or three successive focus points of a given light ray. GRIN lenses that are commonly used typically have a 1/4 pitch which is the distance between a collimating plane and a focus point. This means that when a focused beam is applied to one end of the GRIN lens at a focus point, a collimated output beam will be supplied at the other end of the GRIN lens at a collimating plane, and conversely when a collimated beam is applied to one end of the GRIN lens at a collimating plane, a focused output will be supplied at the opposite end of the GRIN lens at the focus point. In this manner a focused beam from one optical fiber may be expanded into a collimated beam in one GRIN lens into the second GRIN lens, where it may be converted back to a refocused beam and directed to another optical fiber to complete the communication path.
GRIN lens are manufactured in the form of elongated GRIN rod lens segments which extend for a multiple number of pitch lengths. Since the GRIN rod lens is commonly employed as a 1/4 pitch element, the GRIN rod lens segments are sliced in such elements for typical applications. In the present invention the GRIN rod lens segments are used to span the distance between the optical taps of an optical bus even though a multiple number of pitch lengths may be required of the GRIN rod lens segments.